Address Offset =7H, DLAB =X, READ/WRITE
This 8 bit read/write register has no effect on the operation of the Serial Port. It is intended as a scratchpad register to
be used by the programmer to hold data temporarily.
6.5.2 PROGRAMMABLE BAUD RATE GENERATOR (AND DIVISOR LATCHES DLH, DLL)
The Serial Port contains a programmable Baud Rate Generator that is capable of dividing the internal PLL clock by any
divisor from 1 to 65535. The internal PLL clock is divided down to generate a 1.8462MHz frequency for Baud Rates less
than 38.4k, a 1.8432MHz frequency for 115.2k, a 3.6864MHz frequency for 230.4k and a 7.3728MHz frequency for
460.8k. This output frequency of the Baud Rate Generator is 16x the Baud rate. Two 8 bit latches store the divisor in 16
bit binary format. These Divisor Latches must be loaded during initialization in order to insure desired operation of the
Baud Rate Generator. Upon loading either of the Divisor Latches, a 16 bit Baud counter is immediately loaded. This
prevents long counts on initial load. If a 0 is loaded into the BRG registers the output divides the clock by the number 3.
If a 1 is loaded the output is the inverse of the input oscillator. If a two is loaded the output is a divide by 2 signal with a
50% duty cycle. If a 3 or greater is loaded the output is low for 2 bits and high for the remainder of the count. The input
clock to the BRG is a 1.8462 MHz clock.
Table 30 shows the baud rates.
6.5.3 EFFECT OF THE RESET ON REGISTER FILE
The Reset Function Table (Table 31) details the effect of the Reset input on each of the registers of the Serial Port.
6.5.4 FIFO INTERRUPT MODE OPERATION
When the RCVR FIFO and receiver interrupts are enabled (FCR bit 0 = "1", IER bit 0 = "1"), RCVR interrupts occur as
follows:
A. The receive data available interrupt will be issued when the FIFO has reached its programmed trigger level; it is
cleared as soon as the FIFO drops below its programmed trigger level.
B. The IIR receive data available indication also occurs when the FIFO trigger level is reached. It is cleared when the
FIFO drops below the trigger level.
C. The receiver line status interrupt (IIR=06H), has higher priority than the received data available (IIR=04H) interrupt.
D. The data ready bit (LSR bit 0) is set as soon as a character is transferred from the shift register to the RCVR FIFO. It
is reset when the FIFO is empty.
When RCVR FIFO and receiver interrupts are enabled, RCVR FIFO timeout interrupts occur as follows:
A. A FIFO timeout interrupt occurs if all the following conditions exist:
-
-
At least one character is in the FIFO.
The most recent serial character received was longer than 4 continuous character times ago. (If 2 stop bits are
programmed, the second one is included in this time delay).
-
-
The most recent CPU read of the FIFO was longer than 4 continuous character times ago.
This will cause a maximum character received to interrupt issued delay of 160 msec at 300 BAUD with a 12 bit
character.
B. Character times are calculated by using the RCLK input for a clock signal (this makes the delay proportional to the
baudrate).
C. When a timeout interrupt has occurred it is cleared and the timer reset when the CPU reads one character from the
RCVR FIFO.
D. When a timeout interrupt has not occurred the timeout timer is reset after a new character is received or after the
CPU reads the RCVR FIFO.
When the XMIT FIFO and transmitter interrupts are enabled (FCR bit 0 = "1", IER bit 1 = "1"), XMIT interrupts occur as
follows:
A. The transmitter holding register interrupt (02H) occurs when the XMIT FIFO is empty; it is cleared as soon as the
transmitter holding register is written to (1 of 16 characters may be written to the XMIT FIFO while servicing this
interrupt) or the IIR is read.
B. The transmitter FIFO empty indications will be delayed 1 character time minus the last stop bit time whenever the
following occurs: THRE=1 and there have not been at least two bytes at the same time in the transmitter FIFO since
the last THRE=1. The transmitter interrupt after changing FCR0 will be immediate, if it is enabled.
SMSC DS – LPC47S45x
Page 70 of 259
Rev. 07/09/2001
DATASHEET
SMSC [ SMSC CORPORATION ]
